In the course of some secondary oil recovery operations, water is injected through an injection well to sweep or drive oil toward an adjacent production well. A serious problem that can arise in such an operation is that the water preferentially moves through permeable strata in the formation and bypasses oil contained in less permeable strata. This narrowly focussed water movement is commonly referred to as "fingering". As a result of fingering, the "sweep efficiency" of many water-swept operations fall far short of what is sought.
Another water movement problem associated with oil recovery operations is referred to as "coning". When an oil well is being produced, water present in a stratum underlying the oil zone can "cone" upwardly and enter the well bore. As the difference in viscosity between the oil and water is usually significant, the water tends to move more easily through the rock or sand matrix adjacent the well bore. As a result, this flow of water excludes the oil from the well bore, which is undesirable.
Because of these problems, there is an ongoing search for an effective means for preventing the movement of water through certain zones or strata associated with an oil reservoir.
Various techniques have been applied in the past for this purpose. In general, the techniques involve plugging the high permeability strata with some fluidic material that remains in place and diverts water movement to the less permeable zones.
In connection with this approach, it is desirable:
that the plugging agent be `selective`, in the sense of concentrating in the high permeability strata; PA1 and that it be adapted to create an effective plug that extends deeply enough along the longitudinal extent of the high permeability zone so that the water flow cannot quickly bypass it and re-enter the zone. PA1 injecting bacterial spores into a permeable stratum to be plugged; PA1 then injecting a solution (brine) which is capable of substaining spore viability while being inadequate to induce spore germination; PA1 and then injecting nutrient solution to induce spore germination and bacterial proliferation. PA1 that umb could be produced from naturally-occurring species of deep groundwater bacteria; or PA1 that umb were sufficiently non-adhesive to penetrate the formation and be evenly distributed therethrough; or PA1 that the umb could be effectively resuscitated in situ without "skin plugging"; or PA1 that the produced biofilm would be effective to plug the stratum. PA1 injecting into the stratum ultramicrobacteria having a diameter in the range of about 0.2 to about 0.4 .mu.m; and PA1 injecting a specific nutrient controlled solution into the stratum to substantially uniformly resuscitate said ultramicrobacteria to the vegetative state and cause them to produce biofilm functional to effect plugging of said stratum. PA1 the ultramicrobacteria penetrate deeply and are distributed generally uniformly in the reservoir matrix-the difficulty with "skin plugging" is resolved; PA1 the ultramicrobacteria return to the vegetative state with injection of a relatively non-specific nutrient; and PA1 saline-resistant bacteria which are indigenous to the subterranean reservoir can be isolated, starved, injected, and evenly resuscitated in situ to provide effective plugging.
One of the methods which has been explored for the purpose of plugging a subterranean permeable zone involves the use of bacteria. Live bacteria in the vegetative state, when injected into a formation, can form adherent microcolonies on the surfaces of the pores and channels in the rock or sand matrix. These colonies produce exopolysaccharides that coalesce to form a confluent biofilm. This biofilm functions to impede aqueous flow through said pores and channels.
Laboratory studies have shown that bacteria biofilm can be effective to effectively seal a simulated reservoir matrix or core formed of fused glass beads. This is disclosed in a paper entitled `Bacterial Fouling in a Model Core System` by J. C. Shaw et al in Applied and Environmental Microbiology, March, 1985, pages 693-701.
This paper further disclosed that when bacterial cultures were passed through a cylindrical fused-glass-bead core, the build-up of a thick biofilm took place at the inlet end of the core, whereas bacterial colonization of surfaces was very sparse in the lower areas of the core. Stated otherwise, the bacteria tended to quickly seal the inlet end of the core. This has been referred to as "skin plugging" and this result is referred to again below.
Additionally, it has been shown that when parallel reservoir cores of differing permeability were simultaneously subjected to the injection with a bacterial plugging agent, the more permeable pathway was first plugged. Stated otherwise, plugging with bacteria is selective of the permeable zone. This was disclosed in U.S. Pat. No. 4,558,739 issued to McInerney et al.
The McInerney patent went on to teach a process embodiment which is of particular interest with respect to the present invention. More particularly, the patent disclosed:
The McInerney process was designed to emplace the spores deeply into the formation. Spores were used because they are small and non-adhesive in nature. The brine was used to displace them deeply into the rock or sand matrix. And the nutrient was used to resuscitate the emplaced spores and induce them to produce biofilm to plug the formation channels.
However, the McInerney process was subject to certain disadvantages.
As bacterial spores, which are metabolically inert spherical cells, are of 1 .mu.m diameter, size constraints restrict the penetration thereof to rocks having a permeability of greater than 1 darcy. In a typical reservoir, there usually exist "fingering" zones, having a permeability less than 1 darcy, which require sealing off.
Further, in order to be successfully returned to the vegatative state, a species-specific nutrient is required or must be developed for each type of spore. Additionally, only a relatively small number of classes of Gram-positive bacteria exhibit spore-forming capability.
These factors limit the use of spores for plugging purposes.
Digressing somewhat, by way of background, to the field of marine microbiology, it has been known that, in a low nutrient environment, the cells of certain bacterial strains undergo significant reductions in cell size and morphological transformations during progressive cell divisions. These reduced-sized cells formed under a starvation regime are defined as `ultramicrobacteria` (umb) or `ultramicrocells`. The diameters of ultramicrobacteria range from about 0.2 .mu.m to about 0.4 .mu.m.
The isolation of ultramicrobacteria from deep ocean water was first discolsed by J. A. Novitsky and R. Y. Morita in 1977 in an article entitled `Survival of a Psychrophilic Marine Vibrio under Long-Term Nutrient Starvation` in Applied Environmental Microbiology 33:635-641.
Subsequent experimental work has demonstrated that ultramicrobacteria can be prepared in the laboratory by simulating the starvation conditions found in low nutrient environments. It has further been observed that the ultramicrobacteria, although in a dormant condition, remain viable during starvation. Further, the dormant condition of some starved microorganisms has been demonstrated to be reversible. The supply of nutrient to the starved cells rapidly produces an increase in cell size, growth, cell division and a return to the original cell configuration. Stated otherwise, once fed, the starved cells may return to the vegetative adherent biofilm-forming state.
Applicants postulated that ultramicrobacteria had a better potential, because of their small size and lack of glycocalyx coating, for penetrating deeply into a relatively "tight" formation to effect plugging thereof, than had bacterial spores or live vegetative bacteria.
However, it will be readily appreciated that at this stage, although the response of marine and soil organisms to starvation and resuscitation had been explored, the responses of microorganisms from other environments were not understood.
It was not predictable: